This invention is born out of a need for a process to destroy the hazardous wastes, including explosives, fuels, propellants, solvents, and other inorganic and organic materials produced by the military and by industry, without the need for polluting the atmosphere, such as do open burning and open detonation. Also required is a disposal means which does not generate or transform the host materials into other compounds whose safety is either unknown or whose disposal is equally as difficult as the
Waste treatment companies around the world are confronting decreasing waste disposal options and accompanying rising costs. For example, sludge ocean dumping in the United States was eliminated in 1988, land filling, and incineration is receiving greater scrutiny due to increasingly stringent state and federal regulations, and public sentiment to reduce pollution such as pathogens, heavy metals, methane gas, and a host of other noxious materials has been on the rise for decades. The disappearance of available land fill sites and the tightening of disposal regulations are driving up disposal costs. Incineration, requiring oxidation of waste at high temperatures with high volumes of air, followed by separation of the effluent gases from the produced ash and the entrained particulate matter, becomes involved, complicated.
Various methods of destroying waste are being employed today to avoid the use of unacceptable disposal techniques. Sludge refuse, for example, is being treated by lime encapsulation, drying and in-vessel composting and similar technologies. New technologies are being developed to destroy waste or at least render the waste safe for disposal. Among those technologies showing promise is supercritical wet oxidation (SCWO).
In recent years, this new method of supercritical water oxidation, has started to be developed. The process achieves substantially complete oxidation of waste by using considerably more compact equipment, thus becoming an excellent candidate for elimination of waste, even on site. Supercritical water oxidation also has the advantage of producing a clean water product suitable for process recycle, thereby facilitating waste minimization
Supercritical water oxidation (SCWO) raises the water borne waste to a pressure greater than 3,200 psia, and to temperatures greater than 702.degree. F. Under these conditions, water acts as an excellent solvent for both organic compounds and air. The oxygen in the air oxidizes the organic matter converting them to the harmless inert products of oxidation-carbon dioxide and water. Many researchers have demonstrated the effectiveness and efficiencies of supercritical water oxidation in destroying organic waste demonstrating destruction efficiencies reaching virtually 100%.
The solvent properties of supercritical water are different from those of liquid water in that non-polar lipophilic compounds are soluble and salts are less soluble. The majority of hazardous wastes which it is desired to destroy will be soluble in supercritical water. Because complete mixing is possible in the single-phase supercritical region, reaction kinetics are not diffusion limited. In contrast, in two-phase systems, concentration gradients across phase boundaries limit the extent to which compounds can be destroyed.
It is expected that the supercritical oxidation of the present invention will result in conversion of hazardous materials to relatively benign substances and/or to substances which are easily treatable such that they are rendered fit for ultimate disposal into the environment. The products of the inventive process are expected to be primarily CO, CO.sub.2, H.sub.2 O, N.sub.2, N.sub.2 O, H.sub.2, and certain salts. Though oxidation in the medium of supercritical water is similar to combustion, it takes place at lower temperatures than incineration and thus is expected to produce only trace amounts of NO.sub.x, thus solving the significant air pollution problem of NO.sub.x emission. Though the process will produce N.sub.2 O, which is a "greenhouse effect" gas, N.sub.2 O is considered to be a much less serious problem than NO.sub.x. In addition, N.sub.2 O in the atmosphere will not tend to form NO.sub.x but will tend to break down into N.sub.2 and O.sub.2.
The inventive process is contained and thus its effluents can be completely controlled, in contrast to open burning. Because water is the reaction medium, the process can be used for a variety of wastes containing water or for contaminated water. There appear to be no technological upper or lower limit to the concentrations of waste material which can be destroyed. The process can be applied to difficult problems such as cleaning contaminated soil from a polluted site, destruction of armaments, such as solid fuel in a missile, and various types of radioactive waste.
The prior art is replete with means to implement wet oxidation of wastes. Supercritical oxidation is described in U.S. Pat. Nos. 4,338,199, and 4,543,190, issued both to Modell. U.S. Pat. No. 4,822,497, to Hong et al., discloses high destruction efficiencies and is typical of the supercritical water oxidation designs being commercialized. Most systems described in the literature use either pure oxygen or oxygen in air. Others use liquid oxidant such as hydrogen peroxide as taught by Welch et al in U.S. Pat. No. 4,861,497. U.S. Pat. No 5,133,877, to Rofer et al., describes a process for treating waste without the need for an oxidant material. U.S. Pat. No. 5,591,415, to Dassel, et al. describes a pressure vessel containing a reactor whose walls provide a barrier between the harsh chemical environment in the reactor and the pressure vessel itself.
However, nearly all of the current approaches to applying this method are continuous feed, or steady-state flow reactors. That is, materials are constantly being added to, and removed from, the reactor, in such a way as to avoid buildup of either reactants or products. These types of reactors are large, expensive and complex. They also tend to be fixed facilities due to their size limiting their most effective deployment to only those sites with major cleanup inventories.
What is need therefore is a simpler method for treating much smaller quantities of waste. In particular, a mobile, or semi-mobile, batch reactor is needed for application to widely dispersed or remote waste sites